chapter 11 - preboiler and boiler corrosion

7/30/2019 Chapter 11 - Preboiler and Boiler Corrosion

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GE Handbook of Water Treatment

Corrosion is one of the main causes of reduced reliability in steam generating systems. It is estimated that problems due to boiler system

corrosion cost industry billions of dollars per year.

Many corrosion problems occur in the hottest areas of the boiler-the water wall, screen, and superheater tubes. Other common problem

areas include deaerators, feedwater heaters, and economizers.

Methods of corrosion control vary depending upon the type of corrosion encountered. The most common causes of corrosion are

dissolved gases (primarily oxygen and carbon dioxide), under-deposit attack, low pH, and attack of areas weakened by mechanical stress,

leading to stress and fatigue cracking.

These conditions may be controlled through the following procedures:

maintenance of proper pH and alkalinity levels

control of oxygen and boiler feedwater contamination

reduction of mechanical stresses

operation within design specifications, especially for temperature and pressure

proper precautions during start-up and shutdown

effective monitoring and control

CORROSION TENDENCIES OF BOILER SYSTEM COMPONENTS

Most industrial boiler and feedwater systems are constructed of carbon steel. Many have copper alloy and/or stainless steel feedwater

heaters and condensers. Some have stainless steel superheater elements.

Proper treatment of boiler feedwater effectively protects against corrosion of feedwater heaters, economizers, and deaerators. The ASME

Consensus for Industrial Boilers (see Chapter 13) specifies maximum levels of contaminants for corrosion and deposition control in boiler

systems.

The consensus is that feedwater oxygen, iron, and copper content should be very low (e.g., less than 7 ppb oxygen, 20 ppb iron, and 15

ppb copper for a 900 psig boiler) and that pH should be maintained between 8.5 and 9.5 for system corrosion protection. In order to

minimize boiler system corrosion, an understanding of the operational requirements for all critical system components is necessary.

Feedwater Heaters

Boiler feedwater heaters are designed to improve boiler efficiency by extracting heat from streams such as boiler water blowdown and

turbine extraction or excess exhaust steam. Feedwater heaters are generally classified as low-pressure (ahead of the deaerator), high-

pressure (after the deaerator), or deaerating heaters.

Regardless of feedwater heater design, the major problems are similar for all types. The primary problems are corrosion, due to oxygen

and improper pH, and erosion from the tube side or the shell side. Due to the temperature increase across the heater, incoming metal

oxides are deposited in the heater and then released during changes in steam load and chemical balances. Stress cracking of welded

components can also be a problem. Erosion is common in the shell side, due to high-velocity steam impingement on tubes and baffles.

Corrosion can be minimized through proper design (to minimize erosion), periodic cleaning, control of oxygen, proper pH control, and the

use of high-quality feedwater (to promote passivation of metal surfaces).

Deaerators

Deaerators are used to heat feedwater and reduce oxygen and other dissolved gases to acceptable levels. Corrosion fatigue at or near

welds is a major problem in deaerators. Most corrosion fatigue cracking has been reported to be the result of mechanical factors, such as

manufacturing procedures, poor welds, and lack of stress-relieved welds. Operational problems such as water/steam hammer can also be

a factor.

Effective corrosion control requires the following practices:

regular monitoring of operation

minimization of stresses during start-up

maintenance of stable temperature and pressure levels

control of dissolved oxygen and pH in the feedwater

regular out-of-service inspection using established nondestructive techniques


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Other forms of corrosive attack in deaerators include stress corrosion cracking of the stainless steel tray chamber, inlet spray valve spring

cracking, corrosion of vent condensers due to oxygen pitting, and erosion of the impingement baffles near the steam inlet connection.

Economizers

Economizer corrosion control involves procedures similar to those employed for protecting feedwater heaters.

Economizers help to improve boiler efficiency by extracting heat from flue gases discharged from the fireside of a boiler. Economizers can

be classified as nonsteaming or steaming. In a steaming economizer, 5-20% of the incoming feedwater becomes steam. Steaming

economizers are particularly sensitive to deposition from feedwater contaminants and resultant under-deposit corrosion. Erosion at tube

bends is also a problem in steaming economizers.

Oxygen pitting, caused by the presence of oxygen and temperature increase, is a major problem in economizers; therefore, it is necessary

to maintain essentially oxygen-free water in these units. The inlet is subject to severe pitting, because it is often the f irst area after the

deaerator to be exposed to increased heat. Whenever possible, tubes in this area should be inspected closely for evidence of corrosion.

Economizer heat transfer surfaces are subject to corrosion product buildup and deposition of incoming metal oxides. These deposits can

slough off during operational load and chemical changes.

Corrosion can also occur on the gas side of the economizer due to contaminants in the flue gas, forming low-pH compounds. Generally,

economizers are arranged for downward flow of gas and upward flow of water. Tubes that form the heating surface may be smooth or

provided with extended surfaces.

Superheaters

Superheater corrosion problems are caused by a number of mechanical and chemical conditions. One major problem is the oxidation of

superheater metal due to high gas temperatures, usually occurring during transition periods, such as start-up and shutdown. Deposits due

to carryover can contribute to the problem. Resulting failures usually occur in the bottom loops-the hottest areas of the superheater tubes.

Oxygen pitting, particularly in the pendant loop area, is another major corrosion problem in superheaters. It is caused when water is

exposed to oxygen during downtime. Close temperature control helps to minimize this problem. In addition, a nitrogen blanket and

chemical oxygen scavenger can be used to maintain oxygen-free conditions during downtime.

Low-Pressure Steam and Hot Water Heating Systems

Hot water boilers heat and circulate water at approximately 200F. Steam heating boilers are used to generate steam at low pressures,

such as 15 psig. Generally, these two basic heating systems are treated as closed systems, because makeup requirements are usually

very low.

High-temperature hot water boilers operate at pressures of up to 500 psig, although the usual range is 35-350 psig. System pressure must

be maintained above the saturation pressure of the heated water to maintain a liquid state. The most common way to do this is to

pressurize the system with nitrogen. Normally, the makeup is of good quality (e.g., deionized or sodium zeolite softened water). Chemical

treatment consists of sodium sulfite (to scavenge the oxygen), pH adjustment, and a synthetic polymer dispersant to control possible irondeposition.

The major problem in low-pressure heating systems is corrosion caused by dissolved oxygen and low pH. These systems are usually

treated with an inhibitor (such as molybdate or nitrite) or with an oxygen scavenger (such as sodium sulfite), along with a synthetic polymer

for deposit control. Sufficient treatment must be fed to water added to make up for system losses, which usually occur as a result of

circulating pump leakage. Generally, 200-400 ppm P-alkalinity is maintained in the water for effective control of pH. Inhibitor requirements

vary depending on the system.

Electric boilers are also used for heating. There are two basic types of electric boilers: resistance and electrode. Resistance boilers

generate heat by means of a coiled heating element. High-quality makeup water is necessary, and sodium sulfite is usually added to

remove all traces of dissolved oxygen. Synthetic polymers have been used for deposit control. Due to the high heat transfer rate at the

resistance coil, a treatment that precipitates hardness should not be used.

Electrode boilers operate at high or low voltage and may employ submerged or water-jet electrodes. High-purity makeup water is required.

Depending on the type of system, sodium sulfite is normally used for oxygen control and pH adjustment. Some systems are designed with

copper alloys, so chemical addition must be of the correct type, and pH control must be in the range suitable for copper protection.

TYPES OF CORROSION

Corrosion control techniques vary according to the type of corrosion encountered. Major methods of corrosion control include maintenance

of the proper pH, control of oxygen, control of deposits, and reduction of stresses through design and operational practices.

Galvanic Corrosion

Galvanic corrosion occurs when a metal or alloy is electrically coupled to a different metal or alloy.

The most common type of galvanic corrosion in a boiler system is caused by the contact of dissimilar metals, such as iron and copper.

These differential cells can also be formed when deposits are present. Galvanic corrosion can occur at welds due to stresses in heat-


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affected zones or the use of different alloys in the welds. Anything that results in a difference in electrical potential at discrete surface

locations can cause a galvanic reaction. Causes include:

scratches in a metal surface

differential stresses in a metal

differences in temperature

conductive deposits

Figure 11-1. Simplified corrosion cell for iron in water.

A general illustration of acorrosion cell for iron in the presence of oxygenis shown in Figure 11-1. Pitting of boiler tube banks has been

encountered due to metallic copper deposits. Such deposits may form during acid cleaning procedures if the procedures do not completely

compensate for the amount of copper oxides in the deposits or if a copper removal step is not included. Dissolved copper may be plated

out on freshly cleaned surfaces, establishing anodic corrosion areas and forming pits, which are very similar to oxygen pits in form and

appearance. This process is illustrated by the following reactions involving hydrochloric acid as the cleaning solvent.

Magnetite is dissolved and yields an acid solution containing both ferrous (Fe+) and ferric (Fe+) chlorides (ferric chlorides are very

corrosive to steel and copper)

Fe3O4 + 8HCl FeCl2 + 2FeCl3 + 4H2O

magnetite hydrochloric acidferrous

chloride

Ferric

chloridewater

Metallic or elemental copper in boiler deposits is dissolved in the hydrochloric acid solution by the following reaction:

FeCl3 + Cu CuCl + FeCl2

ferric

chloridecopper

cuprous

chloride

Ferrous

chloride

Once cuprous chloride is in solution, it is immediately redeposited as metallic copper on the steel surface according to the following

reaction:

2CuCl + Fe FeCl2 + 2Cu0

cuprous

chlorideiron

ferrous

chloridecopper oxide

Thus, hydrochloric acid cleaning can cause galvanic corrosion unless the copper is prevented from plating on the steel surface. A

complexing agent is added to prevent the copper from redepositing. The following chemical reaction results:

FeCl3 + Cu +Complexing

Agent FeCl2 + CuCl


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ferric

chloridecopper

ferrous

chloride

cuprous

chloride

complex

This can take place as a separate step or during acid cleaning. Both iron and the copper are removed f rom the boiler, and the boiler

surfaces can then be passivated.

In most cases, the copper is localized in certain tube banks and causes random pitting. When deposits contain large quantities of copper

oxide or metallic copper, special precautions are required to prevent the plating out of copper during cleaning operations.

Caustic Corrosion

Concentration of caustic (NaOH) can occur either as a result of steam blanketing (which allows salts to concentrate on boiler metal

surfaces) or by localized boiling beneath porous deposits on tube surfaces.

Figure 11-2. Boiler system tube shows high pH gouging.

Caustic corrosion (gouging) occurs when caustic is concentrated and dissolves the protective magnetite (Fe3O4 ) layer. Iron, in contact

with the boiler water, forms magnetite and the protective layer is continuously restored. However, as long as a high caustic concentration

exists, the magnetite is constantly dissolved, causing a loss of base metal and eventual failure (see Figure 11-2).

Steam blanketing is a condition that occurs when a steam layer forms between the boiler water and the tube wall. Under this condition,

insufficient water reaches the tube surface for efficient heat transfer. The water that does reach the overheated boiler wall is rapidlyvaporized, leaving behind a concentrated caustic solution, which is corrosive.

Porous metal oxide deposits also permit the development of high boiler water concentrations. Water flows into the deposit and heat

applied to the tube causes the water to evaporate, leaving a very concentrated solution. Again, corrosion may occur.

Caustic attack creates irregular patterns, often referred to as gouges. Deposition may or may not be found in the affected area.

Boiler feedwater systems using demineralized or evaporated makeup or pure condensate may be protected from caustic attack through

coordinated phosphate/pH control. Phosphate buffers the boiler water, reducing the chance of large pH changes due to the development

of high caustic concentrations. Excess caustic combines with disodium phosphate and forms trisodium phosphate. Sufficient disodium

phosphate must be available to combine with all of the free caustic in order to form trisodium phosphate.

Disodium phosphate neutralizes caustic by the following reaction:

Na2HPO4 + NaOH Na3PO4 + H2O

disodium phosphate sodium hydroxide

trisodium

phosphate water


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Figure 11-3. Caustic under-deposit corrosion can be controlled through a coordinatedphosphate/pH program.This results in the prevention ofcaustic buildup beneath deposits or within a crevice where leakage is occurring. Caustic corrosion (and

caustic embrittlement, discussed later) does not occur, because high caustic concentrations do not develop (see Figure 11-3).

Figure 11-4. Coordinated phosphate/pH program controls free caustic and resultantcorrosion.

Figure 11-4 shows thephosphate/pH relationshiprecommended to control boiler corrosion. Different forms of phosphate consume or add

caustic as the phosphate shifts to the proper form. For example, addition of monosodium phosphate consumes caustic as it reacts with

caustic to form disodium phosphate in the boiler water according to the following reaction:

NaH2PO4 + NaOH Na2HPO4 + H2O

monosodium

phosphate

sodium

hydroxide

disodium

phosphatewater

Conversely, addition of trisodium phosphate adds caustic, increasing boiler water pH:

Na3PO4 + H2O Na2HPO4 + NaOH

trisodium phosphate water disodium phosphate sodium


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hydroxide

Control is achieved through feed of the proper type of phosphate to either raise or lower the pH while maintaining the proper phosphate

level. Increasing blowdown lowers both phosphate and pH. Therefore, various combinations and feed rates of phosphate, blowdown

adjustment, and caustic addition are used to maintain proper phosphate/pH levels.

Elevated temperatures at the boiler tube wall or deposits can result in some precipitation of phosphate. This effect, termed "phosphate

hideout," usually occurs when loads increase. When the load is reduced, phosphate reappears.

Clean boiler water surfaces reduce potential concentration sites for caustic. Deposit control treatment programs, such as those based on

chelants and synthetic polymers, can help provide clean surfaces.

Where steam blanketing is occurring, corrosion can take place even without the presence of caustic, due to the steam/magnetite reaction

and the dissolution of magnetite. In such cases, operational changes or design modifications may be necessary to eliminate the cause of

the problem.

Acidic Corrosion

Low makeup or feedwater pH can cause serious acid attack on metal surfaces in the preboiler and boiler system. Even if the original

makeup or feedwater pH is not low, feedwater can become acidic from contamination of the system. Common causes include the

following:

improper operation or control of demineralizer cation units

process contamination of condensate (e.g., sugar contamination in food processing plants)

cooling water contamination from condensers

Acid corrosion can also be caused by chemical cleaning operations. Overheating of the cleaning solution can cause breakdown of the

inhibitor used, excessive exposure of metal to cleaning agent, and high cleaning agent concentration. Failure to neutralize acid solvents

completely before start-up has also caused problems.

In a boiler and feedwater system, acidic attack can take the form of general thinning, or it can be localized at areas of high stress such as

drum baffles, "U" bolts, acorn nuts, and tube ends.

Hydrogen Embrittlement

Hydrogen embrittlement is rarely encountered in industrial plants. The problem usually occurs only in units operating at or above 1,500 psi.

Hydrogen embrittlement of mild steel boiler tubing occurs in high-pressure boilers when atomic hydrogen forms at the boiler tube surface

as a result of corrosion. Hydrogen permeates the tube metal, where it can react with iron carbides to form methane gas, or with other

hydrogen atoms to form hydrogen gas. These gases evolve predominantly along grain boundaries of the metal. The resulting increase in

pressure leads to metal failure.

The initial surface corrosion that produces hydrogen usually occurs beneath a hard, dense scale. Acidic contamination or localized low-pH

excursions are normally required to generate atomic hydrogen. In high-purity systems, raw water in-leakage (e.g., condenser leakage)

lowers boiler water pH when magnesium hydroxide precipitates, resulting in corrosion, formation of atomic hydrogen, and initiation of

hydrogen attack.

Coordinated phosphate/pH control can be used to minimize the decrease in boiler water pH that results f rom condenser leakage.

Maintenance of clean surfaces and the use of proper procedures for acid cleaning also reduce the potential for hydrogen attack.

Oxygen Attack

Without proper mechanical and chemical deaeration, oxygen in the feedwater will enter the boiler. Much is flashed off with the steam; the

remainder can attack boiler metal. The point of attack varies with boiler design and feedwater distribution. Pitting is frequently visible in the

feedwater distribution holes, at the steam drum waterline, and in downcomer tubes.

Oxygen is highly corrosive when present in hot water. Even small concentrations can cause serious problems. Because pits can penetrate

deep into the metal, oxygen corrosion can result in rapid failure of feedwater lines, economizers, boiler tubes, and condensate lines.

Additionally, iron oxide generated by the corrosion can produce iron deposits in the boiler.


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Figure 11-5. Oxygen pitting of a boiler feedwater pipe.

Oxygen corrosion may be highly localized or may cover an extensive area. It is identified by well defined pits or a very pockmarked

surface. The pits vary in shape, but are characterized by sharp edges at the surface. Active oxygen pits are distinguished by a reddish

brown oxide cap (tubercle).Removal of this cap exposes black iron oxide within the pit (see Figure 11-5).

Oxygen attack is an electrochemical process that can be described by the following reactions:

Anode:

Fe Fe2+ + 2e

Cathode:

O2 + H2O + 2e 2OH

Overall:

Fe + O2 + H2O Fe(OH)2The influence of temperature is particularly important in feedwater heaters and economizers. A temperature rise provides enough

additional energy to accelerate reactions at the metal surfaces, resulting in rapid and severe corrosion.

At 60F and atmospheric pressure, the solubility of oxygen in water is approximately 8 ppm. Efficient mechanical deaeration reduces

dissolved oxygen to 7 ppb or less. For complete protection from oxygen corrosion, a chemical scavenger is required following mechanical

deaeration.

Major sources of oxygen in an operating system include poor deaerator operation, in-leakage of air on the suction side of pumps, thebreathing action of receiving tanks, and leakage of undeaerated water used for pump seals.

The acceptable dissolved oxygen level for any system depends on many factors, such as feedwater temperature, pH, flow rate, dissolved

solids content, and the metallurgy and physical condition of the system. Based on experience in thousands of systems, 3-10 ppb of

feedwater oxygen is not significantly damaging to economizers. This is reflected in industry guidelines.

the ASME consensus is less than 7 ppb (ASME recommends chemical scavenging to "essentially zero" ppb)

TAPPI engineering guidelines are less than 7 ppb

EPRI fossil plant guidelines are less than 5 ppb dissolved oxygen

MECHANICAL CONDITIONS AFFECTING CORROSION

Many corrosion problems are the result of mechanical and operational problems. The following practices help to minimize these corrosion

problems:

election of corrosion-resistant metals

reduction of mechanical stress where possible (e.g., use of proper welding procedures and stress-relieving welds)

minimization of thermal and mechanical stresses during operation

operation within design load specifications, without over-firing, along with proper start-up and shutdown procedures

maintenance of clean systems, including the use of high-purity feedwater, effective and closely controlled chemical treatment,

and acid cleaning when required

Where boiler tubes fail as a result of caustic embrittlement, circumferential cracking can be seen. In other components, cracks follow the

lines of greatest stress. A microscopic examination of a properly prepared section of embrittled metal shows a characteristic pattern, with


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cracking progressing along defined paths or grain boundaries in the crystal structure of the metal (see Figure 11-6). The cracks do not

penetrate the crystals themselves, but travel between them; therefore, the term "intercrystalline cracking" is used.

Good engineering practice dictates that the boiler water be evaluated for embrittling characteristics. An embrittlement detector (described

in Chapter 14) is used for this purpose.

If a boiler water possesses embrittling characteristics, steps must be taken to prevent attack of the boiler metal. Sodium nitrate is a

standard treatment for inhibiting embrittlement in lower-pressure boiler systems. The inhibition of embrittlement requires a definite ratio of

nitrate to the caustic alkalinity present in the boiler water. In higher-pressure boiler systems, where demineralized makeup water is used,embrittling characteristics in boiler water can be prevented by the use of coordinated phosphate/pH treatment control, described previously

under "Caustic Corrosion." This method prevents high concentrations of free sodium hydroxide from forming in the boiler, eliminating

embrittling tendencies.

Caustic Embrittlement

Caustic embrittlement (caustic stress corrosion cracking), or intercrystalline cracking, has long been recognized as a serious form of boiler

metal failure. Because chemical attack of the metal is normally undetectable, failure occurs suddenly-often with catastrophic results.

For caustic embrittlement to occur, three conditions must exist:

the boiler metal must have a high level of stress

a mechanism for the concentration of boiler water must be present

the boiler water must have embrittlement-producing characteristics

Figure 11-6. Caustic stress corrosion cracking (embrittlement) of a boiler tube.Photomicrograph shows intercrystalline cracking.

Where boiler tubes fail as a result of caustic embrittlement, circumferential cracking can be seen. In other components, cracks follow the

lines of greatest stress. A microscopic examination of a properly prepared section of embrittled metal shows a characteristic pattern, with

cracking progressing along defined paths or grain boundaries in the crystal structure of the metal (see Figure 11-6). The cracks do not

penetrate the crystals themselves, but travel between them; therefore, the term "intercrystalline cracking" is used.

Good engineering practice dictates that the boiler water be evaluated for embrittling characteristics. An embrittlement detector (described

in Chapter 14) is used for this purpose.

If a boiler water possesses embrittling characteristics, steps must be taken to prevent attack of the boiler metal. Sodium nitrate is a

standard treatment for inhibiting embrittlement in lower-pressure boiler systems. The inhibition of embrittlement requires a definite ratio of

nitrate to the caustic alkalinity present in the boiler water. In higher-pressure boiler systems, where demineralized makeup water is used,

embrittling characteristics in boiler water can be prevented by the use of coordinated phosphate/pH treatment control, described previously

under "Caustic Corrosion." This method prevents high concentrations of free sodium hydroxide from forming in the boiler, eliminating

embrittling tendencies.

Fatigue Cracking

Fatigue cracking (due to repeated cyclic stress) can lead to metal failure. The metal failure occurs at the point of the highest concentration

of cyclic stress. Examples of this type of failure include cracks in boiler components at support brackets or rolled in tubes when a boiler

undergoes thermal fatigue due to repeated start-ups and shutdowns.


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Thermal fatigue occurs in horizontal tube runs as a result of steam blanketing and in water wall tubes due to frequent, prolonged lower

header blowdown.

Corrosion fatigue failure results from cyclic stressing of a metal in a corrosive environment. This condition causes more rapid failure than

that caused by either cyclic stressing or corrosion alone. In boilers, corrosion fatigue cracking can result from continued breakdown of the

protective magnetite film due to cyclic stress.

Corrosion fatigue cracking occurs in deaerators near the welds and heat-affected zones. Proper operation, close monitoring, and detailed

out-of-service inspections (in accordance with published recommendations) minimize problems in deaerators.

Steam Side Burning

Steam side burning is a chemical reaction between steam and the tube metal. It is caused by excessive heat input or poor circulation,

resulting in insufficient flow to cool the tubes. Under such conditions, an insulating superheated steam film develops. Once the tube metal

temperature has reached 750F in boiler tubes or 950-1000F in superheater tubes (assuming low alloy steel construction), the rate of

oxidation increases dramatically; this oxidation occurs repeatedly and consumes the base metal. The problem is most frequently

encountered in superheaters and in horizontal generating tubes heated from the top.

Erosion

Erosion usually occurs due to excessive velocities. Where two-phase flow (steam and water) exists, failures due to erosion are caused by

the impact of the fluid against a surface. Equipment vulnerable to erosion includes turbine blades, low-pressure steam piping, and heat

exchangers that are subjected to wet steam. Feedwater and condensate piping subjected to high-velocity water flow are also susceptible

to this type of attack. Damage normally occurs where flow changes direction.

METALLIC OXIDES IN BOILER SYSTEMS

Iron and copper surfaces are subject to corrosion, resulting in the formation of metal oxides. This condition can be controlled through

careful selection of metals and maintenance of proper operating conditions.

Iron Oxide Formation

Iron oxides present in operating boilers can be classified into two major types. The first and most important is the 0.0002-0.0007 in. (0.2-

0.7 mil) thick magnetite formed by the reaction of iron and water in an oxygen-free environment. This magnetite forms a protective barrier

against further corrosion.

Magnetite forms on boiler system metal surfaces from the following overall reaction:

3Fe + 4H2O Fe3O4 + 4H2

iron water magnetite hydrogen

The magnetite, which provides a protective barrier against further corrosion, consists of two layers. The inner layer is relatively thick,

compact, and continuous. The outer layer is thinner, porous, and loose in structure. Both of these layers continue to grow due to water

diffusion (through the porous outer layer) and lattice diffusion (through the inner layer). As long as the magnetite layers are left

undisturbed, their growth rate rapidly diminishes.

The second type of iron oxide in a boiler is the corrosion products, which may enter the boiler system with the feedwater. These are

frequently termed "migratory" oxides, because they are not usually generated in the boiler. The oxides form an outer layer over the metal

surface. This layer is very porous and easily penetrated by water and ionic species.

Iron can enter the boiler as soluble ferrous ions and insoluble ferrous and ferric hydroxides or oxides. Oxygen-free, alkaline boiler water

converts iron to magnetite, Fe3O4. Migratory magnetite deposits on the protective layer and is normally gray to black in color.

Copper Oxide Formation

A truly passive oxide film does not form on copper or its alloys. In water, the predominant copper corrosion product is cuprous oxide

(Cu2O). A typical corrosion reaction follows:

8Cu + O2 + 2H2O 4Cu2O + 2H2

copper oxygen water cuprousoxide hydrogen


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Figure 11-7. Model of oxide layers on copper shows thickness of outer oxide layer.

As shown in Figure 11-7,the oxide that develops on the copper surfaces is comprised of two layers. The inner layer is very thin, adherent,

nonporous, and comprised mostly of cupric oxide (CuO). The outer layer is thick, adherent, porous and comprised mainly of cuprous oxide

(Cu2O). The outer layer is formed by breakup of the inner layer. At a certain thickness of the outer layer, an equilibrium exists at which the

oxide continually forms and is released into the water.

Maintenance of the proper pH, elimination of oxygen, and application of metal-conditioning agents can minimize the amount of copper

alloy corrosion.

Metal Passivation

The establishment of protective metal oxide lay-ers through the use of reducing agents (such as hydrazine, hydroquinone, and other

oxygen scavengers) is known as metal passivation or metal conditioning. Although "metal passivation" refers to the direct reaction of the

compound with the metal oxide and "metal conditioning" more broadly refers to the promotion of a protective surface, the two terms are

frequently used interchangeably.

The reaction of hydrazine and hydroquinone, which leads to the passivation of iron-based metals, proceeds according to the following

reactions:

N2H4 + 6Fe2O3 4Fe3O4 + 2H2O + N2

hydrazine hematite magnetite water nitrogen

C6H4(OH)2 + 3Fe2O3 2Fe3O4 + C6H4O2 + H2O

hydroquinone hematite magnetite benzoquinone water

Similar reactions occur with copper-based metals:

N2H4 + 4CuO 2Cu2O + 2H2O + N2

hydrazine cupricoxide cuprousoxide water nitrogen

C6H6O2 + 2CuO Cu2O + C6H4O2 + H2O

hydroquinone cupricoxide cuprousoxide benzoquinone water

Magnetite and cuprous oxide form protective films on the metal surface. Because these oxides are formed under reducing conditions,removal of the dissolved oxygen f rom boiler feedwater and condensate promotes their formation. The effective application of oxygen

scavengers indirectly leads to passivated metal surfaces and less metal oxide transport to the boiler whether or not the scavenger reacts

directly with the metal surface.


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Figure 11-8. Feedwater oxygen, iron and copper levels show dramatic reduction whenhydroquinone-based materials are used instead of hydrazine (data taken during start-ups and excursions).

A significant reduction in feedwater oxygen and metal oxides can occur with proper application of oxygen scavengers (see Figure 11-8).

CORROSION CONTROL FACTORS

Steel and Steel Alloys

Protection of steel in a boiler system depends on temperature, pH, and oxygen content. Generally, higher temperatures, high or low pH

levels, and higher oxygen concentrations increase steel corrosion rates.

Mechanical and operational factors, such as velocities, metal stresses, and severity of service can strongly influence corrosion rates.

Systems vary in corrosion tendencies and should be evaluated individually.

Copper and Copper Alloys

Many factors influence the corrosion rate of copper alloys:

temperature

pH

oxygen concentration

amine concentration

ammonia concentration

flow rate

The impact of each of these factors varies depending on characteristics of each system. Temperature dependence results from faster

reaction times and greater solubility of copper oxides at elevated temperatures. Maximum temperatures specified for various alloys range

from 200 to 300F.

Methods of minimizing copper and copper alloy corrosion include:

replacement with a more resistant metal

elimination of oxygen

maintenance of high-purity water conditions

operation at the proper pH level

reduction of water velocities

application of materials which passivate the metal surfaces

pH Control

Maintenance of proper pH throughout the boiler feedwater, boiler, and condensate systems is essential for corrosion control. Most low-

pressure boiler system operators monitor boiler water alkalinity because it correlates very closely with pH, while most feedwater,

condensate, and high-pressure boiler water requires direct monitoring of pH. Control of pH is important for the following reasons:


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corrosion rates of metals used in boiler systems are sensitive to variations in pH

low pH or insufficient alkalinity can result in corrosive acidic attack

high pH or excess alkalinity can result in caustic gouging/cracking and foaming, with resultant carryover

speed of oxygen scavenging reactions is highly dependent on pH levels

The pH or alkalinity level maintained in a boiler system depends on many factors, such as sys-tem pressure, system metals, feedwater

quality, and type of chemical treatment applied.

Figure 11-9. Iron corrosion product release from carbon steel in boiler feedwater.The corrosion rate of carbon steel at feedwater temperatures approaches a minimum value in the pH range of 9.2-9.6(see Figure 11-9). It

is important to monitor the feedwater system for corrosion by means of iron and copper testing. For systems with sodium zeolite or hot

lime softened makeup, pH adjustment may not be necessary. In systems that use deionized water makeup, small amounts of caustic soda

or neutralizing amines, such as morpholine and cyclohexylamine, can be used.

Figure 11-10. High or low boiler water pH corrodes boiler steel.

In the boiler, either high or low pH increases the corrosion rates of mild steel (see Figure 11-10). The pH or alkalinity that is maintained

depends on the pressure, makeup water characteristics, chemical treatment, and other factors specific to the system.

Figure 11-11. Average copper release as a function of pH shows optimum pH in rangeof 8.8 to 9.2 for different copper-based alloys. (Courtesy of the Electric Power ResearchInstitute.)


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The best pH for protection of copper alloys is somewhat lower than the optimum level for carbon steel. For systems that contain both

metals, the condensate and feedwater pH is often maintained between 8.8 and 9.2 for corrosion protection of both metals. The optimum

pH varies from system to system and depends on many factors, including the alloy used(see Figure 11-11).

To elevate pH, neutralizing amines should be used instead of ammonia, which (especially in the presence of oxygen) accelerates copper

alloy corrosion rates. Also, amines form protective films on copper oxide surfaces that inhibit corrosion.

Oxygen Control

Chemical Oxygen Scavengers. The oxygen scavengers most commonly used in boiler systems are sodium sulfite, sodium bisulfite,

hydrazine, catalyzed versions of the sulfites and hydrazine, and organic oxygen scavengers, such as hydroquinone and ascorbate.

It is of critical importance to select and properly use the best chemical oxygen scavenger for a given system. Major factors that determine

the best oxygen scavenger for a particular application include reaction speed, residence time in the system, operating temperature and

pressure, and feedwater pH. Interferences with the scavenger/oxygen reaction, decomposition products, and reactions with metals in the

system are also important factors. Other contributing factors include the use of feedwater for attemperation, the presence of economizers

in the system, and the end use of the steam. Chemical oxygen scavengers should be fed to allow ample time for the scavenger/oxygen

reaction to occur. The deaerator storage system and the feedwater storage tank are commonly used feed points.

In boilers operating below 1,000 psig, sodium sulfite and a concentrated liquid solution of catalyzed sodium bisulfite are the most

commonly used materials for chemical deaeration due to low cost and ease of handling and testing. The oxygen scavenging property of

sodium sulfite is illustrated by the following reaction:

2Na2SO3 + O2 2Na2SO4

sodiumsulfite

oxygen sodiumsulfate

Theoretically, 7.88 ppm of chemically pure sodium sulfite is required to remove 1.0 ppm of dissolved oxygen. However, due to the use of

technical grades of sodium sulfite, combined with handling and blowdown losses during normal plant operation, approximately 10 lb of

sodium sulfite per pound of oxygen is usually required. The concentration of excess sulfite maintained in the feedwater or boiler water also

affects the sulfite requirement.

Sodium sulfite must be fed continuously for maximum oxygen removal. Usually, the most suitable point of application is the drop leg

between the deaerator and the storage compartment. Where hot process softeners are followed by hot zeolite units, an additional feed is

recommended at the filter effluent of the hot process units (prior to the zeolite softeners) to protect the ion exchange resin and softener

shells.

As with any oxygen scavenging reaction, many factors affect the speed of the sulfite-oxygen reaction. These factors include temperature,

pH, initial concentration of oxygen scavenger, initial concentration of dissolved oxygen, and catalytic or inhibiting effects. The most

important factor is temperature. As temperature increases, reaction time decreases; in general, every 18F increase in temperaturedoubles reaction speed. At temperatures of 212F and above, the reaction is rapid. Overfeed of sodium sulfite also increases reaction rate.

The reaction proceeds most rapidly at pH values in the range of 8.5-10.0.

Certain materials catalyze the oxygen-sulfite reaction. The most effective catalysts are the heavy metal cations with valences of two or

more. Iron, copper, cobalt, nickel, and manganese are among the more effective catalysts.

Figure 11-12. Comparison of the reaction rates of catalyzed sulfite and sodium sulfitewith dissolved oxygen.

Figure 11-12 compares the removal of oxygen using commercial sodium sulfite and a catalyzed sodium sulfite. After 25 seconds of

contact, catalyzed sodium sulfite removed the oxygen completely. Uncatalyzed sodium sulfite removed less than 50% of the oxygen in this

same time period. In a boiler feedwater system, this could result in severe corrosive attack.


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The following operational conditions necessitate the use of catalyzed sodium sulfite:

low feedwater temperature

incomplete mechanical deaeration

rapid reaction required to prevent pitting in the system

short residence time

use of economizers

High feedwater sulfite residuals and pH values above 8.5 should be maintained in the feedwater to help protect the economizer from

oxygen attack.

Some natural waters contain materials that can inhibit the oxygen/sulfite reaction. For example, trace organic materials in a surface supply

used for makeup water can reduce speed of scavenger/oxygen reaction time. The same problem can occur where contaminated

condensate is used as a portion of the boiler feedwater. The organic materials complex metals (natural or formulated catalysts) and

prevent them from increasing the rate of reaction.

Sodium sulfite must be fed where it will not contaminate feedwater to be used for attemporation or desuperheating. This prevents the

addition of solids to the steam.

At operating pressures of 1,000 psig and higher, hydrazine or organic oxygen scavengers are normally used in place of sulfite. In these

applications, the increased dissolved solids contributed by sodium sulfate (the product of the sodium sulfite-oxygen reaction) can become

a significant problem. Also, sulfite decomposes in high-pressure boilers to form sulfur dioxide (SO2) and hydrogen sulfide (H2S). Both of

these gases can cause corrosion in the return condensate system and have been reported to contribute to stress corrosion cracking in

turbines. Hydrazine has been used for years as an oxygen scavenger in high-pressure systems and other systems in which sulfite

materials cannot be used. Hydrazine is a reducing agent that removes dissolved oxygen by the following reaction:

N2H4 + O2 2H2O + N2

hydrazine oxygen water nitrogen

Because the products of this reaction are water and nitrogen, the reaction adds no solids to the boiler water. The decomposition products

of hydrazine are ammonia and nitrogen. Decomposition begins at approximately 400F and is rapid at 600F. The alkaline ammonia does

not attack steel. However, if enough ammonia and oxygen are present together, copper alloy corrosion increases. Close control of the

hydrazine feed rate can limit the concentration of ammonia in the steam and minimize the danger of attack on copper-bearing alloys. The

ammonia also neutralizes carbon dioxide and reduces the return line corrosion caused by carbon dioxide.

Hydrazine is a toxic material and must be handled with extreme care. Because the material is a suspected carcinogen, federally published

guidelines must be followed for handling and reporting. Because pure hydrazine has a low flash point, a 35% solution with a flash point ofgreater than 200F is usually used. Theoretically, 1.0 ppm of hydrazine is required to react with 1.0 ppm of dissolved oxygen. However, in

practice 1.5-2.0 parts of hydrazine are required per part of oxygen.

Figure 11-13. Time/temperature relationship for 90% oxygen removal by hydrazine at apH of 9.5.

The factors that influence the reaction time of sodium sulfite also apply to other oxygen scavengers. Figure 11-13 shows rate of reaction

as a function of temperature and hydrazine concentration. The reaction is also dependent upon pH (the optimum pH range is 9.0-10.0).


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In addition to its reaction with oxygen, hydrazine can also aid in the formation of magnetite and cuprous oxide (a more protective form of

copper oxide), as shown in the following reactions:

N2H4 + 6Fe2O3 4Fe3O4 + N2 + 2H2O

hydrazine hematite magnetite nitrogen water

and

N2H4 + 4CuO 2Cu2O + N2 + 2H2O

hydrazinecupric

oxide

cuprous

oxidenitrogen water

Because hydrazine and organic scavengers add no solids to the steam, feedwater containing these materials is generally satisfactory for

use as attemperating or desuperheating water.

The major limiting factors of hydrazine use are its slow reaction time (particularly at low temperatures), ammonia formation, effects on

copper-bearing alloys, and handling problems.

Organic Oxygen Scavengers. Several organic compounds are used to remove dissolved oxygen from boiler feedwater and condensate.

Among the most commonly used compounds are hydroquinone and ascorbate. These materials are less toxic than hydrazine and can be

handled more safely. As with other oxygen scavengers, temperature, pH, initial dissolved oxygen concentration, catalytic effects, and

scavenger concentration affect the rate of reaction with dissolved oxygen. When fed to the feedwater in excess of oxygen demand or whenfed directly to the condensate, some organic oxygen scavengers carry forward to protect steam and condensate systems.

Hydroquinone is unique in its ability to react quickly with dissolved oxygen, even at ambient temperature. As a result of this property, in ad-

dition to its effectiveness in operating systems, hydroquinone is particularly effective for use in boiler storage and during system start-ups

and shutdowns. It is also used widely in condensate systems.

Hydroquinone reacts with dissolved oxygen as shown in the following reactions:

C6H4(OH)2 + O2 C6H4O2 + H2O

hydroquinone oxygen benzoquinone water

Benzoquinone reacts further with oxygen to form polyquinones:

C6H4O2 + O2 polyquinones

benzoquinone oxygen

These reactions are not reversible under the alkaline conditions found in boiler feedwater and condensate systems. In fact, further

oxidation and thermal degradation (in higher-pressure systems) leads to the final product of carbon dioxide. Intermediate products are low

molecular weight organic compounds, such as acetates.

Oxygen Level Monitoring. Oxygen monitoring provides the most effective means of controlling oxygen scavenger feed rates. Usually, a

slight excess of scavenger is fed. Feedwater and boiler water residuals provide an indication of excess scavenger feed and verify chemical

treatment feed rates. It is also necessary to test for iron and copper oxides in order to assess the effectiveness of the treatment program.

Proper precautions must be taken in sampling for metal oxides to ensure representative samples.

Due to volatility and decomposition, measurement of boiler residuals is not a reliable means of control. The amount of chemical fed should

be recorded and compared with oxygen levels in the feedwater to provide a check on the control of dissolved oxygen in the system. With

sodium sulfite, a drop in the chemical residual in the boiler water or a need to increase chemical feed may indicate a problem. Measures

must be taken to determine the cause so that the problem can be corrected.

Sulfite residual limits are a function of boiler operating pressure. For most low- and medium-pressure systems, sulfite residuals should be

in excess of 20 ppm. Hydrazine control is usually based on a feedwater excess of 0.05-0.1 ppm. For different organic scavengers,

residuals and tests vary.

MONITORING AND TESTING

Effective corrosion control monitoring is essential to ensure boiler reliability. A well planned monitoring program should include the

following:


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proper sampling and monitoring at critical points in the system

completely representative sampling

use of correct test procedures

checking of test results against established limits

a plan of action to be carried out promptly when test results are not within established limits

a contingency plan for major upset conditions

a quality improvement system and assessment of results based on testing and inspections

Monitoring Techniques

Appropriate monitoring techniques vary with different systems. Testing should be performed at least once per shift. Testing frequency may

have to be increased for some systems where control is difficult, or during periods of more variable operating conditions. All monitoring

data, whether spot sampling or continuous, should be recorded.

Boiler feedwater hardness, iron, copper, oxygen, and pH should be measured. Both iron and copper, as well as oxygen, can be measured

on a daily basis. It is recommended that, when possible, a continuous oxygen meter be installed in the feedwater system to detect oxygen

intrusions. Iron and copper, in particular, should be measured with care due to possible problems of sample contamination.

If a continuous oxygen meter is not installed, periodic testing with spot sampling ampoules should be used to evaluate deaerator

performance and potential for oxygen contamination from pump seal water and other sources.

For the boiler water, the following tests should be performed:

phosphate (if used)

P-alkalinity or pH

sulfite (if used)

conductivity

Sampling

It is critical to obtain representative samples in order to monitor conditions in the boiler feedwater system properly. Sample lines,

continuously flowing at the proper velocity and volume, are required. Generally, a velocity of 5-6 ft/sec and a flow of 800-1000 mL/min are

satisfactory. The use of long sample lines should be avoided. Iron and copper sampling should be approached with extreme care because

of the difficulty of obtaining representative samples and properly interpreting results. Trends, rather than individual samples, should be

used to assess results. Copper sampling requires special precautions, such as acidification of the stream. Composite sampling, rather

than spot sampling, can also be a valuable tool to determine average concentrations in a system.

Oxygen sampling should be performed as close to the line as possible, because long residence time in sampling lines can allow the

oxygen scavenger to further react and reduce oxygen readings. Also, if in-leakage occurs, falsely high data may be obtained. Sampling for

oxygen should also be done at both the effluent of the deaerator and effluent of the boiler feedwater pump, to verify that oxygen ingress is

not occurring.

Results and Action Required

All inspections of equipment should be thorough and well documented.

Conditions noted must be compared to data from previous inspections. Analytical results and procedures must be evaluated to ensure that

quality standards are maintained and that steps are taken for continual improvement.Cause-and-effect diagrams(see Figure 11-14) can


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be used either to verify that all potential causes of problems are reviewed, or to troubleshoot a particular corrosion-related problem.

Figure 11-14. Boiler corrosion cause-and-effect diagram shows major types and causesof corrosion.

CORROSION PROTECTION DURING DOWNTIME AND STORAGE

Oxygen corrosion in boiler feedwater systems can occur during start-up and shutdown and while the boiler system is on standby or in

storage, if proper procedures are not followed. Systems must be stored properly to prevent corrosion damage, which can occur in a matter

of hours in the absence of proper lay-up procedures. Both the water/steam side and the fireside are subject to downtime corrosion and

must be protected.

Off-line boiler corrosion is usually caused by oxygen in-leakage. Low pH causes further corrosion. Low pH can result when oxygen reacts

with iron to form hydroferric acid. This corrosion product, an acidic form of iron, forms at water-air interfaces.

Corrosion also occurs in boiler feedwater and condensate systems. Corrosion products generated both in the preboiler section and the

boiler may deposit on critical heat transfer surfaces of the boiler during operation and increase the potential for localized corrosion or

overheating.

The degree and speed of surface corrosion depend on the condition of the metal. If a boiler contains a light surface coating of boiler

sludge, surfaces are less likely to be attacked because they are not fully exposed to oxygen-laden water. Experience has indicated that

with the improved cleanliness of internal boiler surfaces, more attention must be given to protection from oxygen attack during storage.

Boilers that are idle even for short time periods (e.g., weekends) are susceptible to attack.

Boilers that use undeaerated water during start-up and during their removal from service can be severely damaged. The damage takes the

form of oxygen pitting scattered at random over the metal surfaces. Damage due to these practices may not be noticed for many yearsafter installation of the unit.

The choice of storage methods depends on the length of downtime expected and the boiler complexity. If the boiler is to be out of service

for a month or more, dry storage may be preferable. Wet storage is usually suitable for shorter down-time periods or if the unit may be

required to go on-line quickly. Large boilers with complex circuits are difficult to dry, so they should be stored by one of the wet storage

methods.

Dry Storage

For dry storage, the boiler is drained, cleaned, and dried completely. All horizontal and non-drainable boiler and superheater tubes must

be blown dry with compressed gas. Particular care should be taken to purge water from long horizontal tubes, especially if they have

bowed slightly.

Heat is applied to optimize drying. After drying, the unit is closed to minimize air circulation. Heaters should be installed as needed to

maintain the temperature of all surfaces above the dew point.

Immediately after surfaces are dried, one of the three following desiccants is spread on water-tight wood or corrosion-resistant trays:

quicklime-used at a rate of 6 lb/100 ft of boiler volume

silica gel-used at a rate of 17 lb/100 ft of boiler volume

activated alumina-used at a rate of 27 lb/100 ft of boiler volume

The trays are placed in each drum of a water tube boiler, or on the top flues of a fire-tube unit. All manholes, handholes, vents, and

connections are blanked and tightly closed. The boiler should be opened every month for inspection of the desiccant. If necessary, the

desiccant should be renewed.


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Wet Storage

For wet storage, the unit is inspected, cleaned if necessary, and filled to the normal water level with deaerated feedwater.

Sodium sulfite, hydrazine, hydroquinone, or another scavenger is added to control dissolved oxygen, according to the following

requirements:

Sodium sulfite. 3 lb of sodium sulfite and 3 lb of caustic soda should be added per 1000 gal of water contained in the boiler

(minimum 400 ppm P-alkalinity as CaCO3 and 200 ppm sulfite as SO3).

Hydrazine. 5 lb of a 35% solution of hydrazine and 0.1 lb of ammonia or 2-3 lb of a 40% solution of neutralizing amine can be

added per 1000 gal (minimum 200 ppm hydrazine and 10.0 pH). Due to the handling problems of hydrazine, organic oxygen

scavengers are normally recommended.

Hydroquinone. Hydroquinone-based materials are added to achieve approximately 200 ppm as hydroquinone in previously

passivated on-line systems. In new systems, or those considered to have a poorly formed magnetite film, the minimum feed

rate is 400 ppm as hydroquinone. pH should be maintained at 10.0.

No matter which treatment is used, pH or alkalinity adjustment to minimum levels is required.

After chemical addition, with vents open, heat is applied to boil the water for approximately 1 hr. The boiler must be checked for proper

concentration of chemicals, and adjustments made as soon as possible.

If the boiler is equipped with a nondrainable superheater, the superheater is filled with high-quality condensate or demineralized water and

treated with a volatile oxygen scavenger and pH control agent. The normal method of f illing nondrainable superheaters is by back-filling

and discharging into the boiler. After the superheater is filled, the boiler should be filled completely with deaerated feedwater. Morpholine,

cyclohexylamine, or similar amines are used to maintain the proper pH.

If the superheater is drainable or if the boiler does not have a superheater, the boiler is allowed to cool slightly after firing. Then, before a

vacuum is created, the unit is filled completely with deaerated feedwater.

A surge tank (such as a 55-gal drum) containing a solution of treatment chemicals or a nitrogen tank at 5 psig pressure is connected to the

steam drum vent to compensate for volumetric changes due to temperature variations.

The drain between the nonreturn valve and main steam stop valve is left open wide. All other drains and vents are closed tightly.

The boiler water should be tested weekly with treatment added as necessary to maintain treatment levels. When chemicals are added,

they should be mixed by one of the following methods:

circulate the boiler water with an external pump

reduce the water level to the normal operating level and steam the boiler for a short time

If the steaming method is used, the boiler should subsequently be filled completely, in keeping with the above recommendations.

Although no other treatment is required, standard levels of the chemical treatment used when the boiler is operating can be present.

Boilers can be protected with nitrogen or another inert gas. A slightly positive nitrogen (or other inert gas) pressure should be maintained

after the boiler has been filled to the operating level with deaerated feedwater.

Storage of Feedwater Heaters and Deaerators

The tube side of a feedwater heater is treated in the same way the boiler is treated during storage. The shell side can be steam blanketed

or flooded with treated condensate.

All steel systems can use the same chemical concentrations recommended for wet storage. Copper alloy systems can be treated with half

the amount of oxygen scavenger, with pH controlled to 9.5.

Deaerators are usually steam or nitrogen blanketed; however, they can be flooded with a lay-up solution as recommended for wet lay-upof boilers. If the wet method is used, the deaerator should be pressurized with 5 psig of nitrogen to prevent oxygen ingress.

Cascading Blowdown

For effective yet simple boiler storage, clean, warm, continuous blowdown can be distributed into a convenient bottom connection on an

idle boiler. Excess water is allowed to overflow to an appropriate disposal site through open vents. This method decreases the potential for

oxygen ingress and ensures that properly treated water enters the boiler. This method should not be used for boilers equipped with

nondrainable superheaters.

Cold Weather Storage


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In cold weather, precautions must be taken to prevent freezing. Auxiliary heat, light firing of the boiler, cascade lay-up, or dry storage may

be employed to prevent freezing problems. Sometimes, a 50/50 water and ethylene glycol mixture is used for freeze protection. However,

this method requires that the boiler be drained, flushed, and filled with fresh feedwater prior to start-up.

Disposal of Lay-up Solutions

The disposal of lay-up chemicals must be in compliance with applicable federal, state, and local regulations.

Fireside Storage

When boilers are removed from the line for extended periods of time, f ireside areas must also be protected against corrosion.

Fireside deposits, particularly in the convection, economizer, and air heater sections, are hygroscopic in nature. When metal surface

temperatures drop below the dew point, condensation occurs, and if acidic hygroscopic deposits are present, corrosion can result.

The fireside areas (particularly the convection, economizer, and air heater sections) should be cleaned prior to storage.

High-pressure alkaline water is an effective means of cleaning the fireside areas. Before alkaline water is used for this purpose, a rinse

should be made with fresh water of neutral pH to prevent the formation of hydroxide gels in the deposits (these deposits can be very

difficult to remove).

Following chemical cleaning with a water solution, the fireside should be dried by warm air or a small fire. If the boiler is to be completely

closed up, silica gel or lime can be used to absorb any water of condensation. As an alternative, metal surfaces can be sprayed or wiped

with a light oil.

If the fireside is to be left open, the metal sur-faces must be maintained above the dew point by circulation of warm air.

chapter 11 - preboiler and boiler corrosion

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